1. Field of the Invention
This invention relates to a chuck device of a machine tool, and more particularly to a chuck device having two collets that can be driven to operate synchronously.
2. Description of the Related Art
Referring to FIG. 1, a conventional hydraulic chuck device includes a machine frame 30, in which a fixed seat 40, a hydraulic cylinder 50, an inner positioning unit 60, a bearing unit 70, a connecting seat 80, a first oil path control unit 90, a second oil path control unit 95, a driving seat 100, a transmission member 110, a pull rod 120, a front cap 130, a rear cap 140, a front collet 150, a rear collet 160, a front cover 170, and a rear cover 180 are provided. When hydraulic oil is fed into an oil hole 45 in the fixed seat 40, it flows into a hole 89 to push a driving ring 102 of the driving seat 100 so that the driving seat 100 pushes the front cap 130 to move forwardly. Hence, the front collet 150 is activated to hold a workpiece (not shown) to be machined. Since the pull rod 120, which is a single rigid body, is connected between the front and rear collets 150, 160, the front and rear collets 150, 160 can open and close in a synchronous manner, so that operation of the front collet 150 affects adversely smooth operation of the rear collet 160. As a result, the rear collet 160 cannot hold firmly another workpiece (not shown) to be machined. Furthermore, due to the presence of the pull rod 120, the opening degrees of the front and rear collets 150, 160 are the same at a time. In a situation where the front and rear collets 150, 160 hold respectively two portions of a workplace to be machined, which have different diameters, if one of the two portions of the workplace is held firmly within one of the front and rear collets 150, 160, the other of the two portions of the workpiece cannot be held firmly within the other of the front and rear collets 150, 160. Consequently, when the chuck device is operated at a high speed, workpiece sway is caused, thereby affecting adversely the machining precision. Further, the front and rear collets 150, 160 cannot be used to machine two workpieces having different diameters at a time so that, when the workpieces to be machined have different diameters, the production capacity may be reduced.